Two boys on skateboards are facing each other. they push off, the first boy (mass: 43.8 kg) traveling backward at 2.1 m/s. if the second boy is now traveling at 3.09 m/s, what is his mass?

4r-134a enters the condenser of a residential heat pump at 800 kpa and 50°c at a rate of 0.022 kg/s and leaves at 750 kpa subcooled by 3°c. the refrigerant enters the compressor at 200 kpa superheated by 4°c determine (a) the isentropic efficiency of the compressor, (b) the rate of heat supplied to the heated room, and (c) the cop of the heat pump. also, determine (d) the cop and rate of heat supplied to the heated room if this heat pump operated on the ideal vapor-compression cycle between the pressure limits of 200 and 800 kpa. (0.757, 4.37 kw, 5.12, 6.18, 3.91 kw)

The sun's declination is 0° at the a. summer and winter solstice b. summer and winter equinox c. vernal and autumnal solstice d. vernal and autumnal equinox

How are elastic and inelastic collisions different? give two or more ways. your answer should have at least 2 sentences.